Crude overhead system
design considerations
Proper crude unit overhead system design is important when building a new unit or
revamping an existing one to process different crudes. Overhead condenser designs should
meet the specific processing objectives, such as distillate yield, run length and energy recovery
Tony Barletta and Steve White
Process Consulting Services
P
rocess
flow
schemes
and
equipment design play a major
role in atmospheric crude column
and condenser system corrosion and
fouling. There are two major types of
overhead systems — single- and twodrum — but there are many different
process flow schemes and several
exchanger
configurations.
Some
overhead systems experience severe
corrosion and fouling that increase the
atmospheric column operating pressure,
reduce the distillate yield or require
exchanger bundle changes at intervals
less than the normal four- to six-year
turnarounds. Some crude columns
have to be shut down to remove salts
deposited on the internals.
290
Raw
crude
250
155
C.W.
110
Single drum
Sour
water
Corrosion and fouling
The atmospheric crude column and
overhead system are exposed to a
number of corrosive species, depending
on the crude type, slop oil processing
and desalter operation. Desalter
operation is an integral part of overhead
system corrosion and the two cannot be
separated. Calcium and magnesium
chloride salts hydrolyse in the
atmospheric heater, creating HCl. When
processing difficult-to-desalt crudes such
as Venezuelan extra-heavy, diluted
bitumen blends and conventional
crudes such as Doba, salt content
entering the heater can be high, with
large amounts of HCl produced. The
most common sources of corrosion and
fouling are the presence of strong acid
formed from the HCl in the condensers
and salts deposited from the reaction of
ammonia or amine neutraliser and
HCl that leads to fouling and underdeposit corrosion.
Process operating conditions that
influence corrosion and fouling include
the crude column overhead temperature,
reflux temperature, overhead vapour
water content, crude temperature and
water wash. Since the process flow
scheme
and
equipment
design
determine the localised temperature and
location of the initial water condensation
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Wet gas
Naphtha
product
Temperature, °F
Figure 1 Single-drum system
on the exchanger tube wall or inside the
column, corrosion and fouling cannot
be separated from the process flow
scheme and operating conditions. Low
crude temperatures entering the crude
column overhead vapour exchanger
reduce the tube wall temperatures below
the water dewpoint, facilitating strong
acid formation. Exchanger tube wall
temperature is not measured; it must be
calculated based on stream rates and
properties as well as the exchanger
design parameters.
Crude oils that are difficult to desalt
such as diluted bitumen are a processing
challenge, because they pose corrosion
and fouling problems in the crude
overhead system as well as the top of the
crude column. In some cases, the crude
column fouling and corrosion can limit
unit run length. Corrosion and fouling
inside the atmospheric column are
caused by low temperature reflux, in
conjunction with the presence of HCl,
NH3 and amines. When column internal
fouling and corrosion are likely, twodrum overhead systems are needed to
avoid low temperatures that cause
localised (shock) water condensation
inside the column.
Water absorbs the HCl, NH3 and
amines, and once the water vapourises
as reflux flowing down the column
heated salts are deposited. These salts
will continue to accumulate, causing a
higher pressure drop. Eventually, they
must be removed or column capacity
will degrade, requiring a shutdown.
Online water washing and the injection
of chemical dispersant are common
methods used to remove the salts.
Increasingly common is the presence of
amine-based H2S scavengers that form
very corrosive salts at the top of the
column when the temperature drops
below 275°F (135°C). Design and
operation of the condensing system are
an integral part of managing the rate of
PTQ Q3 2007 53
310
Raw
crude
155
250
C.W.
Wet gas
110
Product
(cold) drum
Reflux
(hot) drum
Sour
water
Naphtha
product
Temperature, °F
Figure 2 Two-drum system — full-range naphtha product
Crude oil
out
Overhead
vapour in
Tube wall below
water dewpoint
80
Crude oil
in
t flange
Outlet
Temperature, °F
Figure 3 Cold crude — tube wall temperature below water dewpoint
fouling and corrosion at the top of
the column.
Overhead system exchanger designs
are numerous. Typically, horizontal
exchangers are used with condensing on
either the tube or shell side of the
exchangers. Shell-side condensing is
most common, even though the
likelihood of fouling and corrosion
increases because it is more difficult to
effectively water wash or chemically
treat the areas around the baffles, where
low velocity prevents chemical or water
wash from reaching the exchanger
tubes. Salts and corrosion products
deposit in these regions, facilitating
under-deposit corrosion.
Tube-side condensing is increasingly
preferred because it makes it easier to
water wash and chemically treat. Vertical
tube-side condensation has the benefit
of being self-draining and it is relatively
easy to distribute wash water and filming
and neutralise chemicals to minimise
fouling.
Crude overhead systems
Crude column overhead systems
condense naphtha product and column
reflux, or a portion of it. Some of this
heat is recoverable. Consequently,
many overhead systems exchange part
of the condensing heat with raw crude.
Overhead systems use either a single
drum (Figure 1), where product and
reflux are withdrawn at approximately
110°F (43°C), or two drums (Figure 2),
where reflux is withdrawn from the
first hot drum and product from the
second cold drum. Often, these drums
are referred to as reflux or hot and
product or cold drums. The reflux
drum operates as high as 280°F (138°C)
with no water present, or as low as
190°F (88°C) with a water phase. Some
systems force water condensation at
the inlet of the condensing side of the
exchanger to dilute the acid strength,
thus avoiding corrosion. Forced water
condensation results in a lowertemperature (approximately 190°F/
88°C) hot drum, increasing the
likelihood of shock water condensation,
corrosion and salt formation inside the
column. There are many factors
involved with selecting a single- or
two-drum system.
Two-drum systems were originally
conceived to recover more heat, but
they also cost more. They have much
higher reflux temperatures than a single
drum too, preventing or minimising
shock water condensation in the
column. Higher energy recovery is
possible because all the reflux heat is
recoverable against crude, whereas only
a portion of this heat is hot enough to
recover in a single-drum system. Single
drums cool reflux and product to the
same low temperature. Figures 1 and 2
show typical operating temperatures.
Assuming all heat below 250°F
(121°C) is lost to air and water cooling,
the two-drum system recovers all the
reflux heat, whereas the single-drum
system cannot because the reflux
temperature is 110°F (43°C), the same as
the product. Reflux heat between 250–
110°F (121–43°C) is lost to air and water
with the single drum. Moreover, the
two-drum system has 15–20°F (8–11°C)
higher temperature overhead vapour
because the column overhead has a
higher endpoint than the same stream
feeding a single drum. The hot drum is
essentially the top theoretical stage in
the column in the single-drum system.
The reflux drum can be operated dry
or wet. Operating the crude versus
overhead exchanger without water wash
requires a higher crude temperature
entering the exchanger as well as control
of the hot drum temperature. Strong
acid formation occurs at the location
where water initially condenses. When
no water is added to the overhead
vapour entering the exchanger, the
exchanger tube wall temperature must
be kept above the water dewpoint. A
small amount of water in the presence
of HCl forms a strong acid. Maintaining
the tube wall temperature above the
water dewpoint typically requires crude
inlet temperatures in the range of 160°F
(71°C), depending on crude viscosity
and other factors affecting the exchanger
heat-transfer coefficient. Charging cold
crude directly from storage to the hot
drum exchanger ensures the tube wall
temperature is below the water dewpoint
(Figure 3), resulting in high corrosion
rates when using carbon steel. Crude
temperature is the controlling variable,
but the reflux drum temperature must
also be controlled high enough to
maintain the tube wall temperature
above the water dewpoint.
Forced water condensation in front of
the hot drum exchanger prevents strong
acid formation. Large amounts of water
are circulated from the boot to the front
of the exchanger, forcing water
condensation and diluting the acid
(Figure 4). Neutraliser is used to maintain
the pH in an acceptable range. Moreover,
the water dissolves the neutralised salts,
preventing deposition and underdeposit corrosion. It also reduces the
exchanger inlet temperature to
54 PTQ Q3 2007 www.eptq.com
approximately 225°F (107°C) and results
in outlet temperatures of approximately
190°F (88°C), depending on the amount
of heat recovered. Forced water
condensation reduces the reflux
temperature below the water dewpoint
inside the crude column, which can lead
to corrosion and fouling when
processing many synthetic and heavy
crude oils and slop oils. Avoiding shock
water condensation requires a reflux
drum temperature of 230°F (110°C) or
higher to prevent internals and vessel
wall corrosion (Figure 5) and fouling
from salts deposition.
Water
wash
155
Raw
crude
C.W.
Reflux
(hot) drum
Wet gas
110
Product
(cold) drum
Sour
water
Single or two drums?
A single-drum system costs a lot less, but
it has the disadvantage of cold 100–
125°F (38–52°C) reflux. Even though the
column overhead temperature is above
the water dewpoint, the cold reflux
causes localised water condensation on
the top tray. Strong acids form when the
water absorbs the HCl, causing high
corrosion rates when the column vapour
HCl content is high. In the presence of
NH3 or amine, high HCl content causes
salt to deposit on the trays, typically two
to three trays from the top of the
column. Moreover, when desalting
heavy crudes, it is not uncommon to
have reasonable desalter operation with
intermittent rag layer formation,
resulting in periodic brine (salt and
water) carryover from the desalter.
During these events, large amounts of
hydrolysable salts flow to the heater and
end up in the overhead of the column.
When cold reflux is present, large
quantities of salt accumulate inside the
column. Single-drum systems with cold
reflux, in conjunction with poor
desalting or intermittent brine carryover,
lead to fouling and corrosion inside
the column.
Naphtha
product
Figure 4 Two-drum system — forced water condensation
Overhead vapour
to condensers
270
Reflux
110
Internal reflux
temperature
increases as it
flows down
tower
Localised
temperature
on top tray
below water
dewpoint
Salt lays down
once water vapourises
Temperature, °F
Figure 5 Shock condensation inside the column — corrosion and fouling
Single-drum systems
Single-drum systems separate the
naphtha product, reflux, water and gas
from the overhead condenser outlet.
Cold reflux is returned to the
atmospheric column, naphtha product
to the saturate gas plant or stabiliser,
sour water to the desalter or sour water
stripper, and gas to the overhead
compressor. Water condensation is
often forced at the inlet of the crude
overhead vapour exchanger by
circulating water wash (Figure 6). This
helps avoid fouling and corrosion in
the exchangers, but it does not address
shock water condensation inside the
column.
Forcing water condensation in front
of the first exchanger dilutes the acid
strength. Water is circulated to the
exchanger inlet at a high enough rate to
ensure 30–50% excess water above the
theoretical dewpoint. Furthermore,
neutralising amines are used to control
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Water wash
Raw
crude
Forced water
condensation
Wet gas
C.W.
Single drum
Sour
water
Naphtha
product
Figure 6 Single-drum system — forced water condensation
PTQ Q3 2007 57
Temperature control of
hot drum – bypass crude
Raw
crude
C.W.
TC
Wet gas
Reflux
(hot) drum
Product
(cold) drum
Sour
water
Naphtha
product
Figure 7 Reflux drum temperature control
to maintain the overhead vapour
temperature control. The hot drum
product is pumped to the cold drum
exchanger inlet so it can be cooled to
the product drum temperature before
routing to the downstream unit. The
flow scheme also needs the capability
to provide column reflux from the cold
drum when the amount of heat
removal before the hot drum is
insufficient to meet the reflux rate.
Operating the reflux drum wet or dry
continues to be a debate. When
processing bitumen upgrader crudes
from Venezuela or Canada and other
heavy crudes, it is essential to maintain
the column reflux above the water
dewpoint; otherwise, severe corrosion
and fouling will occur inside the
column. Furthermore, the crude
column overhead vapour temperature
must be operated higher than normal
to prevent corrosive salt formation
from the amine H2S scavengers or
other amines.
Exchanger design
Crude oil
out
Overhead
vapour in
Areas in front and
behind baffles
prone to fouling
and corrosion
t flange
Crude oil
in
Difficult to water wash
around baffles
Outlet
Figure 8 Horizontal exchanger, shell-side condensing
the
water
pH.
Forcing
water
condensation decreases the exchanger
inlet temperature to approximately
225°F (107°C), with the resultant outlet
even lower, causing a reflux temperature
below the water dewpoint for the top
tray. Single-drum systems always force
water condensation in the top of the
column. Processing crudes that are
difficult to desalt leads to fouling and
corrosion inside the column and in
external systems such as top
pumparound exchangers.
Two-drum systems
Two-drum systems have a reflux and
product drum. Overhead vapour from
the crude column is exchanged with
crude, generating hot reflux, vapour
and often a liquid product. Vapour
from the hot drum requires an
additional heat exchanger surface to
condense the product and water prior
to the cold drum. Sometimes, an
additional crude exchanger service is
used on the hot drum vapour to recover
some of the naphtha product heat. In
the crude preheat train, this exchanger
is located upstream of the crude
overhead exchanger, raising the crude
temperature into the exchanger. This
increases the tube wall temperatures in
the exchanger, which is one of the
primary factors influencing the
corrosion rate. Hot drum temperatures
operate between 280–170°F (138–77°C),
depending on whether the drum is
operated dry or with forced water
condensation at the inlet.
Preventing water condensation
inside the column requires a reflux
temperature above the water dewpoint,
which is approximately 225°F (107°C)
when producing full-range naphtha
overhead. The hot drum must be
operated dry to ensure the reflux is
always above the column top tray water
dewpoint. Operating dry requires
temperature control that bypasses part
of the crude around the exchanger to
ensure the exchanger outlet temperature
(Figure 7) is high enough to prevent
water formation. Hot drum flow
schemes need provisions to make a
liquid product when heat removal
condenses more reflux than is required
The crude overhead vapour exchanger
design is critical to meeting reliability
and heat-recovery objectives. Important
design decisions include shell- or tubeside condensing, horizontal or vertical
design and shell-side baffle type.
Determining which fluid is on the shell
or tube side of the exchanger is the first
decision. With heavy crude on the tube
side, the heat-transfer coefficients are
very low, requiring a high surface area.
Tube-side crude velocities should be
maintained above 5–6 ft/s (1.52–1.8 m/
s); otherwise, rapid fouling occurs. Yet,
even these moderate velocities generate
an extremely high tube-side pressure
drop because crude in the tube is in the
laminar regime. Hence, as crude oils get
heavier, crude must be on the shell side
and
column
overhead
vapour
condensation will be on the tube side.
Tube-side condensation has the benefit
of eliminating areas around the baffles
where the velocity is low and corrosion
products and salts tend to accumulate
(Figure 8). Furthermore, the outside tube
surfaces close to the baffles do not get
properly treated with chemical or water
wash even when forced condensation is
used. Tube-side condensation is
preferred even for light crude oils.
The exchangers can be oriented
horizontally or vertically with tube-side
condensing. Horizontal exchangers
have been typically used. Vertical onepass tube-side exchangers have the
advantage of being self-draining,
eliminating areas where corrosive low
pH water can accumulate (Figure 9).
Vertical
exchangers
have
the
disadvantage of higher exchanger cost
and higher installation cost because
additional platform levels and structure
are needed to accommodate the vertical
58 PTQ Q3 2007 www.eptq.com
design. However, vertical exchangers are
gaining favour and are preferred.
When crude is on the shell side,
minimising fouling requires proper
baffle selection and design. A standard
vertical baffle has dead areas that simply
cannot be avoided even with optimised
baffle spacing and cut ratios. Hence,
exchangers such as the proprietary
Helixchanger, which use quadrantshaped segmented baffles to generate
nearly true plug flow, should be
employed to minimise fouling, optimise
pressure drop and maximise the heattransfer coefficient throughout the run.
Overhead
system
exchanger
metallurgy ranges from carbon steel to
upgrades such as dual alloy stainless, all
the way to exotic materials like titanium
and hastelloy. Carbon steel is by far the
most common material, but it is subject
to rapid corrosion when strong acid is
formed or solids deposit, generating
under-deposit corrosion. When carbon
steel is used, the exchanger tube wall
temperature must be above the water
dewpoint or forced water condensation
is needed to ensure sufficient water is
present to avoid strong acid formation
or salt formation. Both hastelloy and
titanium permit much lower pH water
than carbon steel, but even these
materials will corrode.
Overhead system design
Overhead condenser designs should
meet the specific processing objectives.
When feeding light, easy-to-desalt crude
oils, a single-drum system using carbon
steel exchangers with forced water
condensation is the lowest-cost design.
Maximising energy recovery requires a
two-drum system. Determining whether
the hot drum should use forced water
condensation or not depends on
whether crude column fouling is likely.
When processing difficult-to-desalt
crudes that are likely to form salts inside
the column, it is necessary to pull fullrange naphtha overhead using a twodrum system with the hot drum
operating dry. When operating dry, to
minimise corrosion in the crude
overhead vapour exchanger before the
hot drum, the system must be designed
to prevent the exchanger tube wall from
operating below the water dewpoint and
possibly
upgraded
metallurgy.
Furthermore, the hot drum needs a
control system with the capability of
bypassing crude the exchanger to
maintain drum temperature. This
sometimes reduces the hot drum liquid
rate below the reflux rate needed to
control the naphtha endpoint. The flow
scheme needs to be designed with the
flexibility to reflux some cold drum
liquid to the column (Figure 10).
The system shown in Figure 10 has a
dry reflux drum with temperature
control and forced water condensation
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in the raw crude, fin-fan and cooling
water exchangers in front of the cold
drum. The hot drum temperature is
maintained by bypassing part of the
crude around the exchanger, which
increases the hot drum vapour rate and
temperature leaving the drum. The cold
drum system forces water condensation
by circulating water from the boot. The
hot drum can either produce a liquid
product that is routed to cold drum
crude exchanger, or the cold drum can
supply a small amount of reflux to the
crude column when the hot drum heat
removal is insufficient to meet reflux
requirements. This design supplies
reflux to the crude column at
temperatures that are always well above
the column top tray water dewpoint.
This prevents fouling and corrosion
inside the column. When processing
slops containing amines or diluted
bitumen containing amine scavenger,
the hot exchanger metallurgy will need
to be upgraded due to the formation of
corrosive salts.
Column overhead
vapour
Crude out
Crude in
Conclusions
Proper crude unit overhead system
design is important when building a
new unit or modifying an existing
system to process different crudes.
Heavier, lower-priced crudes continue
to be the main feedstock for new unit
construction and the revamp of existing
units in certain refining regions. While
these crudes are lower priced, they also
are more difficult to process. The crude
overhead system is an integral part of
unit reliability and profitability.
Tony Barletta is a chemical engineer with
Figure 9 Vertical exchanger, one-pass
tube-side condensing
Process Consulting Services in Houston,
Texas, USA.
Email: tbarletta@revamps.com
Steve White is a chemical engineer with
Process Consulting Services in Houston,
Texas, USA. Email: swhite@revamps.com
Temp. control of
hot drum–bypass crude
Cold raw
crude
TC
C.W.
Wet gas
Sour
water
Naphtha
product
When required
Figure 10 Overhead system design: two-drum system for meeting specific requirements
PTQ Q3 2007 59